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UCC39411DG4 Datasheet(PDF) 5 Page - Texas Instruments |
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UCC39411DG4 Datasheet(HTML) 5 Page - Texas Instruments |
5 / 22 page www.ti.com APPLICATION INFORMATION OPERATION UCC29411, UCC29412 UCC29413, UCC39411 UCC39412, UCC39413 SLUS245E – MARCH 2000 – REVISED JULY 2005 A detailed block diagram of the UCC39411 is shown in Figure 1. Unique control circuitry provides high-efficiency power conversion for both light and heavy loads by transitioning between discontinuous and continuous conduction based on load conditions. Figure 2 depicts converter waveforms for the application circuit shown in Figure 3. A single 22µH inductor provides the energy pulses required for a highly efficient 3.3 V converter at up to 200 mW output power. At time t1, the 3.3 V output voltage has dropped below its lower threshold, and the inductor is charged with an on time determined by: tON = 5.5 µs/VIN. For a 1.25 V input and a 22 µH inductor, the resulting peak current is approximately 250 mA. At time t2, the inductor begins to discharge with a minimum off time of approximately 1 ms. Under lightly loaded conditions, the amount of energy delivered in this single pulse satisfies the voltage-control loop, and the converter does not command any more energy pulses until the output again drops below the lower-voltage threshold. At time t3, the VGD supply drops below its lower threshold, but the output voltage is still above its threshold point. This results in an energy pulse to the gate-drive supply at t4. In some cases, a single pulse supplied to VGD is insufficient to raise the VGD voltage level enough to satisfy the voltage loop. Under this condition, multiple pulses are supplied to VGD. Note that when the UCC3941x is servicing VGD only, the IC maintains a discontinuous mode of operation. After time t4, the 3.3 V output drops below its threshold and requests to be serviced once the VGD cycle has completed, which occurs at time t5. Time t6 represents a transition between light load and heavy load. A single energy pulse is not sufficient to force the output voltage above its upper threshold before the minimum off time has expired and a second charge cycle is commanded. Since the inductor current does not reach zero in this case, the peak current is greater than 250 mA at the end of the next charge on time. The result is a ratcheting of inductor current until either the output voltage is satisfied, or the converter reaches its set current limit. At time t7, the gate drive voltage has dropped below its 7 V threshold but the converter continues to service the output because it has higher priority unless VGD drops below about 6.3 V. Between time t7 and t8, the converter reaches its peak current limit. Once the peak current is reached, the converter operates in continuous mode with approximately 60 mA of inductor current ripple. At time t8, the 3.3 V output is satisfied and the converter can service the gate drive voltage, VGD, which occurs at time t9. 5 |
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